This invention relates generally to turbine systems used in power generation, and more particularly to a system and method of providing in-situ incremental reheating to increase thermodynamic efficiency of such turbine systems.
The steam-Rankine power cycle is a standard thermodynamic power cycle that converts heat in to power. As such, the efficiency at which it converts heat to power depends most importantly on the temperature to which steam is raised (higher is better) and the temperature at which low-grade heat is removed from the power cycle (lower is better). It has been the historical practice at steam-electric power plants to expand high temperature steam in a high pressure (HP) turbine then reheat the steam before it expands in an intermediate pressure (IP) turbine, FIG. 1. A handful of power plants have employed double reheat in which steam issuing from the IP turbine is reheated again before being sent to a low-pressure (LP) turbine. By increasing the average temperature of heat addition to the steam working fluid, the overall power cycle efficiency and net plant efficiency is increased.
The use of single reheat steam-Rankine power cycles is standard for steam-electric plants greater than about 150 MWe capacity. The increased steam piping/controls costs and the more laborious start-up and shutdown sequences of operations associated with double reheat have limited its acceptance by power plant developer/owners. State of the art steam-electric power plants employ main steam conditions of up to 4000 psia/1120° F. and single reheat temperature of up to 1120° F.
Steam turbines generally consist of alternating stationary (stator) and rotating (rotator) blades arranged in a circle around the turbine shaft. The stationary blades turn and accelerate the steam flow. The steam momentum is transferred to the rotating blades which turn the turbine shaft and, ultimately, the electric generator. A stator with the rotator following, together, make up a single stage of the turbine. Typical HP and LP steam turbines will have in excess of 10 stages in series.
In the last decade the prospect of increasing steam temperatures to as high as 1400° F. and pressures as high as 5100 psia with single reheat temperatures as high as 1400° F. has been investigated. However, increasing steam temperatures above the state of the art 1120° F. requires the use of high nickel alloys not currently used in common steam-electric power plants. These high nickel alloys are required for producing high pressure main steam and reheat steam in final superheater/reheaters to convey the high pressure/temperature steam from the boiler to the turbine.
Unfortunately, suitable high nickel alloys are likely to cost an order of magnitude more than the steels in state of the art power plants. This has led to the pursuit of alternative ways of conveying the high temperature energy from the boiler to the turbine at pressures lower than the main steam pressure, minimizing the strength requirements and hence material quantities required of these exotic metals.
Accordingly, there remains a need for a system and method of increasing steam temperature without the need for expensive alloys and piping.